1. Field of the Invention
The present invention relates a vapor compression cycle applied to various devices such as air conditioning units, refrigerating machines, and heat pumps, which utilize a coolant (especially, CO2) driven under supercritical conditions at a high side in a closed system.
2. Background Art
In the supercritical vapor compression cycle, a few techniques have been proposed for controlling the high side pressure by adjusting the circulating coolant. An example is shown in Japanese Patent Publication No. Hei 7-18602). This supercritical vapor compression cycle comprises, as shown in FIG. 6, a compressor 100 serially connected to the radiator 110, a countercurrent-type heat exchanger 120, and a throttle valve 130. An evaporator 140, a liquid separator (a receiver) 160, and the low pressure side of the countercurrent heat exchanger 120 are connected so as to communicate each other to an intermediate point between the throttle valve 130 and a inlet 190 of the compressor 100. The receiver 160 is connected to the outlet 150 of the evaporator 150 and the gas phase inlet of the receiver is connected to the countercurrent heat exchanger 120. A liquid phase line (shown by a broken line) from the receiver 160 is connected to a suction line at an optional point between a point 170 located at the front side of the countercurrent-type heat exchanger 120 and a point 180 located at the back side on the heat exchanger 120. The above-described throttle valve 130 changes the residual quantity of the liquid in the receiver 160 for adjusting the high side supercritical vapor pressure. A conventional example shown in FIG. 7 comprises, instead of the receiver, an intermediate liquid reservoir 250, provided with respective valves at both inlet and outlet sides, and a throttle valve 130, connected in parallel with the reservoir 250.
Recently, a new vapor compression refrigerating cycle using CO2 (hereinafter, called the CO2 cycle) is proposed as one alternative for eliminating freon-type coolants. The operation of this CO2 cycle is the same as that of the conventional vapor compression-type refrigerating cycle using freon. That is, operations include, as shown by A-B-C-D-A in FIG. 3 (CO2 Mollier chart), compressing CO2 in the vapor phase (A-B), and cooling the compressed and high temperature vapor phase CO2 by the radiator (gas cooler) (B-C). Then, the operation continues for reducing the pressure of the vapor phase CO2 by the pressure reducing device (C-D), evaporating CO2 separated into two gas-liquid phases (D-A), and cooling the outside fluid by removing the latent heat of vaporization from the outside fluid.
The critical temperature of CO2 is 31xc2x0 C., which is lower than that of conventional freon. Thus, in hot seasons like summer, the temperature of CO2 near the radiator becomes higher than the critical temperature of CO2. Thus, CO2 gas does not condense (the line segment BC does not cross the saturated liquid line). Since the state of the outlet point of the radiator (point C) is determined by the discharge pressure of the compressor and the temperature of CO2 at the radiator outlet and since the CO2 temperature at the radiator outlet is determined by the heat dissipation capacity and the temperature of the outside air (this is not controllable), the temperature of the radiator outlet is substantially uncontrollable. The state at the radiator outlet (point C) becomes controllable by controlling the discharge pressure (pressure at the radiator outlet) of the compressor. That is, in order to preserve a sufficient cooling capacity (the enthalpy difference) when the temperature of the outside air is high like in summer, it is necessary to make the pressure of the radiator outlet high as shown by E-F-G-H-E in FIG. 4.
However, since the discharge pressure of the compressor must be raised in order to raise the radiator outlet pressure, the work of compression done by the compressor (an enthalpy variation A L in the compression process) increases. Thus, if the enthalpy variation A L in the compression process (A-B) is larger than the enthalpy variation A I of the evaporation process (D-A), the performance factor of the CO2 cycle (COP=xcex94I/xcex94L) is lowered. When the relationship between the CO2 pressure at the radiator outlet and the performance factor is calculated with reference to FIG. 3, assuming that the temperature of CO2 at the radiator outlet is 40xc2x0 C., the maximum performance factor is obtained at the pressure P, as shown by the solid line in FIG. 5. Similarly, when the temperature of the CO2 gas at the radiator outlet side is assumed at 30xc2x0 C., the maximum performance factor is obtained at a pressure P2 (approximately 8.0 MPa).
As shown above, when the CO2 temperatures at the radiator outlet and the pressure for obtaining the maximum performance factor are calculated and plotted, the bold solid line xcex7max (hereinafter, called the optimum control line) is yielded. Therefore, in order to operate the CO2 efficiently, it is necessary to control both of the radiator outlet pressure and the CO2 temperature at the radiator outlet so as to be correlated as shown by the optimum control line xcex7max.
However, since the above described supercritical vapor compression cycle (FIGS. 6 and 7) is not the system in which the radiator outlet pressure (high side pressure) is controlled in correspondence to the coolant temperature at the radiator outlet, and the cooling efficiency at the radiator is not sufficiently high, there is room to improve cooling efficiency. Another problem arises that, when the circulating coolant quantity must be controlled to correspond to the control of the high side pressure (a larger amount of circulating coolant is necessary as the high side pressure increases), the opening of the throttle valve must be adjusted manually whenever it is necessary, which is a time consuming operation and requires much experience.
The present invention is realized in order to overcome the above problems, and thus, it is therefore an objective of the present invention to provide a supercritical vapor compression cycle, provided with a gas cooler (radiator) having an improved cooling efficiency, and capable of automatically controlling the necessary circulating coolant quantity in accordance with an adjustment of the high side pressure.
According to a first aspect of the present invention, a supercritical vapor compression cycle is provided by serially connecting a compressor, a gas cooler, a diaphragm device, and an evaporator by a pipe so as to constitute a closed circuit to be operated at a supercritical pressure at the high pressure side in vapor compression cycle, which comprises: a pressure control valve, provided between said gas cooler and said diaphragm device, for controlling a pressure at an outlet of said gas cooler; a reservoir, through which a pipe from the outlet of said evaporator penetrates, for storing a liquid coolant; and a communication pipe for communicating between the bottom of said reservoir and the pipe connecting said pressure control valve with said diaphragm device.
According to the second aspect, the supercritical vapor compression cycle according to the first aspect further comprises an intercooler for executing heat change between the liquid coolant which has passed through said evaporator and the gas coolant which has passed through said evaporator, wherein said pressure control valve is disposed at a pipe from the outlet of said intercooler.
According to the third and fourth aspects, in a supercritical vapor compression cycle according to the first or the second aspect, the coolant used in the cycle is carbon dioxide.